1. Field of the Invention
The invention relates to mechanisms for initiating a control function responsive to local interruption of movement of an article along a path, such as an automatic interruption mechanism which is suitable for a magnetic tape apparatus for recording and/or reproducing signals on a moving magnetic tape (hereinafter referred to as a tape recorder stop mechanism); and more particularly to a tape recorder having a capstan for transport of the magnetic tape with an accurately defined speed, rotatable about a first axis of rotation and while driving the magnetic tape; a pressure roller which co-operates with the capstan and is rotatable about a second axis of rotation which is parallel to the first axis of rotation; means for driving the capstan; and a stop mechanism which, after being activated, automatically interrupts the drive of the magnetic tape when the tape comes to a standstill, at least locally.
More generally, apparatus for which such mechanisms are useful have:
(A) A FIRST ROTATABLY DRIVEN COMPONENT WHICH DURING OPERATION OF THE APPARATUS PERFORMS A ROTARY MOVEMENT AND ALSO KEEPS ROTATING AFTER THE ARTICLE SUCH AS A TAPE HAS COME TO A LOCAL STANDSTILL AT LEAST DURING THE TIME WHICH IS NECESSARY FOR THE ACTIVATION OF THE INTERRUPTION OR OTHER CONTROL MECHANISM,
(B) A SECOND ROTATABLE COMPONENT, WHICH DURING OPERATION OF THE APPARATUS PERFORMS A ROTARY MOVEMENT ONLY DURING NORMAL TRANSPORT OF THE ARTICLE SUCH AS TAPE AND WHOSE STOPPAGE CONSTITUTES A SIGNAL FOR THE CONTROL FUNCTION THAT THE ARTICLE HAS COME TO A LOCAL STANDSTILL,
(C) A DIFFERENTIAL MECHANISM, INCLUDING A SLIP COUPLING, PRIMARY AND SECONDARY COUPLING SECTIONS WHICH ARE ROTATABLE RELATIVE TO EACH OTHER AND WHICH ARE ROTABLE WITH GIVEN TRANSMISSION RATIOS BY THE FIRST AND SECOND ROTATABLE COMPONENTS, AND A ZERO MEMBER MOVABLE IN A FIRST AND ALSO IN A SECOND DIRECTION OF MOVEMENT AND BIASED TOWARD ONE DIRECTION, THESE DIRECTIONS OF MOVEMENT BEING DEPENDENT ON THE DIFFERENCE IN SPEED OF ROTATION OF THE ROTARY MOVEMENTS (THE SLIP) BETWEEN THE PRIMARY AND SECONDARY COUPLING SECTIONS; AND EACH OF THE COUPLING SECTIONS BEING SO DRIVEN AND THE ZERO MEMBER BEING SO LOADED THAT DURING TRANSPORT OF THE ARTICLE THE ZERO MEMBER IS CONTINUOUSLY SUBJECTED TO A FORCE IN ITS FIRST DIRECTION OF MOVEMENT WHILE DURING LOCAL INTERRUPTION OF ARTICLE MOVEMENT THE ZERO MEMBER IS DRIVEN IN ITS SECOND DIRECTION OF MOVEMENT,
(D) STOP MEANS WHICH LIMIT MOVEMENT OF THE ZERO MEMBER IN ITS FIRST DIRECTION OF MOVEMENT, AND
(E) A CONTROL DEVICE SUCH AS A SWITCHING SYSTEM WHICH IS ENGAGED BY THE ZERO MEMBER FOR TURNING OFF THE APPARATUS DRIVE AFTER THE ZERO MEMBER HAS MOVED IN ITS SECOND DIRECTION OF MOVEMENT.
There is a growing interest on the part of consumers in the presence of an automatically operating mechanism which interrupts or switches off the apparatus when the end of a magnetic tape is reached, in particular in the case of magnetic-tape cassette equipment. In cassette equipment which is suitable for playing back the cassette in both directions the drive means may be changed over automatically after the interruption, so that automatically continued reproduction of a program in the cassette is possible. In equipment which is not suitable for automatic change-over of the direction of movement the interest in an automatic stop feature for the apparatus mainly arises from the wish to prevent unnecessary wear and possible damage to the magnetic tape and, in the case of a battery-powered units, running the batteries down.
2. Description of the Prior Art
In known automatic interruption mechanisms of the type mentioned above the first rotatably driven component during operation and which also keeps rotating after the local stoppage of the tape transport, generally consists of a spindle such as a capstan which is driven directly by a drive motor. The second rotatable component, which rotates only during tape transport and whose stopping constitutes a signal that the tape has come to a local standstill, is generally constituted by a reel disc or a pressure roller.
When the switching mechanism responds to stoppage of the rotation of a reel disc, this does not necessarily mean that the magnetic tape has come to a standstill in other parts of the apparatus at the same time. For example, stoppage of the turntable may be the result of jamming of a take-up reel. If that occurs and the magnetic-tape transport continues at the location of the capstan continues, loops are formed in the magnetic tape in the cassette, so that the magnetic tape and the cassette, may be damaged.
When pressure roller speed is sensed, and the switching mechanism responds to stoppage of the magnetic tape at the location of the capstan, the pressure roller should have a width which is smaller or not much greater than the width of the magnetic tape, because otherwise the pressure roller is likely still to be driven by the capstan after the magnetic tape has come to a standstill.
An interruption mechanism of the first type is known from German Patent Specification No. 1,286,773. In this known interruption mechanism two central or sun gear wheels of a planetary gear arrangement are journalled on a spindle. The zero member consists of a lever pivotally journalled at one end about the same spindle and carrying two planet wheels at its free end, each engaging a respective sun gear. One of the two sun gears is driven by a capstan through an intermediate gear wheel, the other central or sun gear being driven by a reel disc, which in its turn is driven by a drive motor, through a friction coupling. The sun gear spindle is mounted on a pivotable lever. The transmission ratio of the planet wheels has been selected so that even at the lowest speed of the reel disc the central gear wheel which is driven thereby pushes the zero lever against a stop in its first direction of movement, which is accompanied by slippage in the friction coupling. When the speed of the reel disc becomes zero because the end of the magnetic tape has been reached, the zero member is moved in its second direction of movement and thus opens a contact, so that the current supply to the drive motor is discontinued.
An interruption mechanism of the second type is known from German Offenlegungsschrift No. 2,332,475. In this mechanism the primary coupling section consists of a spindle which is parallel to the capstan, the spindle circumference being resiliently pressed against the pressure roller. The pressure roller then serves as secondary coupling section, the slip between the coupling sections occurring between the circumference of the pressure roller and the circumference of the primary coupling section. The primary coupling section is journalled the zero member, which is in a U-shaped bracket pivoted near the end of its limbs about an axis in line with the axis of rotation of the pressure roller. On the capstan a gear pinion is located which, via an idler wheel, drives a gear wheel mounted on the primary coupling section, with a 1:1 transmission ratio. The diameter of the primary coupling section has been selected so that during transport of the tape the primary coupling section has a slightle lower circumferential speed than the pressure roller. The slip between these two components results in a frictional force which presses the U-shaped bracket, or zero member, against a stop. As soon as the magnetic tape comes to a standstill, the pressure roller also stops. The capstan, however, continues to rotate so that the primary coupling section also remains driven. As a result of this the spindle performs a rolling movement over the circumference of the pressure roller, so that the zero member is moved in its second direction of movement and moves a catch so that the catch comes within range of a cam which is mounted on a flywheel which is coupled to the capstan. The cam exerts an impact force on the catch, so that a latching slide in the tape recorder control mechanism is unlatched and a drive motor is switched off.
In these and similar interruption mechanisms the zero member is pressed against a first stop during transport of the tape by a frictional force in the slip coupling. In practice, frictional forces which act between two components which are movable relative to each other are never constant, but exhibit fluctuations. Owing to the mutual coupling of the components of the differential mechanism and the drive motor the fluctuations in friction result in speed variations which are eventually transmitted to the magnetic tape. This may give rise to undesired variations in the frequency of the signal which is reproduced or recorded. The force with which the zero member is moved in its second direction upon termination of the transport of the magnetic tape is determined by the friction in the friction coupling. This force is always small, because during transport of the magnetic tape a small frictional force is desirable, so as to avoid unnecessary loss of energy and also to avoid unnecessary variations in friction of unacceptable magnitude. The force available for switching over and/or switching off the apparatus is consequently small.